1. Field of the Invention
The present invention relates to a wireless communication system. More particularly, the present invention relates to an apparatus and a method for receiving a signal using a rake receiver and an equalizer.
2. Description of the Related Art
A transmission signal from a transmission end of a wireless communication system arrives at a reception end after experiencing a fading phenomenon with various delays under a multi-wireless path channel environment. The reception end may use a rake receiver and an equalizer in order to recover an original signal from a reception signal.
The reception end of the wireless communication system may demodulate signals received at different times via multipaths for respective paths, remove time delay, combine the signals of the respective paths, and decode the signals. Here, a unit for demodulating the multipath signals independently and combining the multipath signals is referred to as a rake receiver. The rake receiver includes a plurality of fingers for demodulating multipath signals for respective paths, and a multipath combiner for removing delay of signals demodulated for the respective paths, and combining them. Here, each finger of the rake receiver is considered as a 1-tap equalizer and regards a path maintained by each finger as one memory-free channel to channel-compensate for a signal received via the path. In contrast, the equalizer regards a multipath as a channel having one memory and channel-compensates for a signal received via the multipath.
Ideally, when data processing performances of the rake receiver and the equalizer are compared with each other, the performance of the equalizer would be better because the equalizer recovers an optimal original signal with consideration of all multipath signals, but a finger of the rake receiver recovers an original signal with consideration of only a signal from one path, so that multipath signals that have not been considered by the relevant finger function as interference. These interference signals are generally referred to as an Inter-Symbol Interference (ISI) or an Inter-Chip Interference (ICI) by multipaths. Therefore, a recovered signal output from each finger is recovered and combined without removing a multipath interference so that a Signal-to-Noise Ratio (SNR) deteriorates compared with an SNR of a recovered signal of the equalizer. That is, use of the equalizer has an advantage in view of reception performance.
However, the equalizer generally is considerably complex compared with that of the rake receiver and consumes more power. A method used for equalizing a channel at a reception end, a method realizing an equalizer as an adaptive filter, or a method for directly determining an inverse matrix of a channel may be used. Alternatively, a method for performing a Fast Fourier Transform (FFT) on a channel estimated by a reception end, and determining an inverse matrix of a channel in a frequency domain may be used. A considerable hardware resource is required for determining an inverse matrix of the channel. More particularly, when the memory size of a channel is large, a required hardware resource increases by geometric progression. Therefore, when realizing an equalizer, a memory size (i.e., a multipath delay length) of a channel is limited.
Under a wireless channel environment where a plurality of relays exist, or under a specific wireless channel environment such as a riverside, a multipath power delay profile exists somewhat long. In this case, when an equalizer having a limited memory size receives a signal, a signal that arrives somewhat late cannot be recovered and an interference signal increases. Accordingly, a performance loss is generated. In contrast, since a rake receiver is less influenced by delay of a wireless channel, when a memory size of a multipath combiner inside the rake receiver is large, a signal is reliably received. Accordingly, a performance loss is not experienced.
As described above, a reception end may recover an original signal from a reception signal using two reception methods (i.e., a method that uses a rake receiver, and a method that uses an equalizer). In using the two reception methods, an advantageous environment and a disadvantageous environment exist.
Therefore, a need exists for an apparatus and method for improving reception performance using a rake receiver and an equalizer in a wireless communication system.